The P2X proteins are ATP gated channels that depolarize cells and also allow calcium to enter. P2X receptors are expressed in virtually every tissue, including neurons and glia of the central and peripheral nervous system, smooth, skeletal and cardiac muscle, cochlear hair cells, platelets, most classes of white blood cells, hepatocytes, and endothelial cells in the lung and gastrointestinal tract. The importance of members of this gene family for normal physiology is apparent from the range of phenotypes that are seen in their absence, Mice in which specific P2X receptors are knocked out show dysfunction in pain perception, ability to void the bladder, gut motility, neuronal control of ejaculation, the ability of the nervous system to monitor the oxygen level in the blood, the ability to fight bacterial infection, and blood clotting. A major problem in the purinergic receptor field is the limited specificity of agonists and antagonists that can be used to alter ATP signaling in vivo. The goal of the experiments described here is to better characterize the molecular mechanisms that allow ATP and allosteric modulators to open P2X receptor channels. The results of these studies should facilitate the development of agents tha't act more specifically on particular receptors. We will use electrophysiological, biochemical, and molecular approaches to study receptors bearing complementary mutations in adjacent or non-adjacent subunits. The specific aims are: Goal 1- To test whether the zinc binding sites that modulate channel activity in P2X2, P2X3, and P2X4 receptors are within or between subunits, and to define residues that participate in these binding sites. We will also define our understanding about the mechanisms by which zinc promotes channel opening in P2X2 receptors. Goal 2 - To test whether the ATP binding site of P2X receptors is within or between subunits and to define additional residues that are exposed in the ATP binding pocket. These experiments will also test the number of molecules of ATP that must be bound in order to open a channel. Goal 3 - To define the molecular movements that are a consequence of zinc or ATP binding to P2X2 receptors. These experiments are of particular relevance to making progress in understanding and treating pain associated with tissue injury, as P2X2 and P2X3 receptors have been implicated as playing essential roles as sensing the damage and signaling the central nervous system. Having a better understanding of the structure of these receptors should allow the development of new treatments for this type of pain, and so greatly ease the suffering of individuals with burns and other injuries that produce persistent pain.